This project is a longitudinal study of the impact of legislation which permits pharmacists to substitute generically equivalent drug products. Data were obtained from a randomly selected sample of prescriptions dispensed in pharmacies in Michigan and Wisconsin, during the year following implementation of the legislation; data for the second and third years will be collected. Data collected include; name, dosage form, strength, and quantity of the drug prescribed and dispensed; number of refills prescribed and dispensed; price of the drug dispensed; payer for the prescription; date filled; whether or not substitution was prohibited, and prescriber information. Data was collected from 116,000 prescriptions in the two states plus information on an additional 1,200 substituted prescriptions from 350,000 prescriptions "scanned" to find evidence of substitution. Newly adopted techniques will produce a larger sample of prescriptions for which substitutions were actually made. Analysis was concentrated on: developing baseline data; defining prescribing and dispensing patterns; estimating the proportion of prescriptions written for multiple versus single source products; estimating the proportion of prescriptions written generically; estimating the proportion of prescriptions paid by third-parties; estimating the extent which physicians prohibited substitution; estimating the proportion of prescriptions for which substitution was possible; and estimating the proportion of prescriptions where substitution occurred. Future analysis will concentrate on various economic impacts of the drug product selection legislation and on estimating the potential savings through drug product selection.